The present invention generally pertains to video signal processing and is particularly directed to improved video signal scrambling and descrambling systems.
One technique for scrambling video signals is described in U.S. Pat. No. 4,070,693 to Shutterly. In accordance with the technique described therein, video information is scrambled by storing a video information line in a memory and then forming a scrambled video information line by retrieving the video information from the memory in a sequence different from the sequence within the line in which the information was stored. The segment at the beginning position of the stored video information line is retrieved at the last position of the scrambled video information line and the segment at the last position of the stored video information line is retrieved at the beginning position of the scrambled video information line. The respective lengths of the retrieved segments are varied from one line to the next in response to a randomly generated coded control signal.
A technique for more thoroughly scrambling video signals is described in U.S. Pat. No. 4,563,702 to Heller and Paik. In accordance with the technique described therein, video information is scrambled by storing a group of video information lines in a memory and then forming a group of scrambled video information lines by retrieving the video information from the memory in a sequence different from the sequence within the group of lines in which the information was stored so as to interchange segments between different video information lines. A scrambled video information line includes segments retrieved from different stored video information lines within each stored group. The segment at the beginning position of one stored video information line is retrieved at the last position of one scrambled video information line and the segment at the last position of another stored video information line is retrieved at the beginning position of the one scrambled video information line. The respective lengths of the retrieved segments are varied from one group of line to the next group in response to a randomly generated encryption keystream. Each group of stored video information lines does not include adjacent video information lines within a video frame, however, since within a video frame, the phase of the chrominance information cycle (as defined by the color subcarrier frequency) of the sequentially odd video information lines is 180 degrees out of phase with the chrominance information cycle of the adjacent sequentially even video information lines; whereby as a result of this out-of-phase relationship, the effect of chrominance leakage varies between the scrambled segments and upon descrambling causes a differential hue shift across a descrambled line or from line to line.
Chrominance leakage is the leakage of a 3.58 MHz local color subcarrier reference signal into the chrominance information. With respect to the sync tips of each video information line, the local color subcarrier reference signal has a different phase relationship between adjacent sequentially odd and even lines of a video frame. Therefore, the phase relationship of the color subcarrier reference signal to the chrominance information is different for the segments of a scrambled video information line respectively derived from different adjacent video information lines; and combining segments from different adjacent lines into a single line causes a hue shift between the different segments when the scrambled signal is descrambled to reconstruct the video signal.
Because of this chrominance leakage phenomenon, each group of stored video information lines within which segments are interchanged in accordance with the more thorough scrambling technique described in said U.S. Pat. No. 4,563,702 includes only sequentially odd video information lines or only sequentially even video information lines, thereby making it necessary to provide sufficient memory capacity for simultaneously storing both a group of odd video information lines and a group of even video information lines.